Study of granular drag force for intruder moving with nonuniform velocities using Discrete Element Method

Abstract

Like in fluids, objects moving in granular materials experience drag force. In granular materials this drag force on the object, or “intruder,” arises from inter-particle friction, as well as the cyclic creation and buckling of force chains within the material. The aim of this work is to investigate whether and how the acceleration of object affects this drag force. This is done by simulating the behavior of granular materials using discrete element method (DEM). Study includes a series of drag tests which involve pulling an intruder through stationary granular packing with a prescribed acceleration profile. The system is parametrized using the dimensionless Froude number 𝐹𝑟 = 2𝑣/√(𝑔𝑅), where 𝑣 is the instantaneous speed of intruder of radius 𝑅. Simulation results evidence the presence of a rate-dependent regime (𝐹𝑟 < 1) where intruder’s acceleration strongly influences the drag force. This behavior is fundamentally different from steady state motion where this regime is governed by friction coefficient of the system and immersion depth of intruder. This behavior is also found in contrast with fluids where an accelerating body in fluid medium experiences an increase in drag due to added mass effect. This peculiar behavior of granular material is determined to be due to the formation of highly anisotropic force chains and their complex dynamics as they evolve under load.